The energy expended in physical activity is an important component of energy balance and body weight. Because of the variety of physical activities that free-living humans undertake, it has been extremely challenging to obtain accurate measurements of either the amount or energy cost of human physical activity. The energy expended in physical activity consists of energy expended in purposeful exercise and the energy expended in non-exercise activity thermo genesis (NEAT). The latter is even more difficult to measure than the former. Since most Americans engage in relatively little purposeful physical activity, the majority of energy expended in physical activity will be through NEAT. The research proposed in this application focuses on improving measurement systems for amount and energy cost of physical activity, with specific techniques proposed for measuring non-exercise physical activity and NEAT. We propose to develop a new high-precision Physical Activity Measurement System (PAMS). PAMS will be developed using off-the-shelf components and macros written in an universally available software program. PAMS will allow physical activity to be accurately measured at low cost and will be easily available to investigators worldwide. In Specific Aim #1, we propose to develop PAMS and validate it using subjects who will be confined in a whole-room indirect calorimeter. In Specific Aim #2, we propose to validate PAMS in free-living subjects. We will then, in Specific Aim #3, utilize PAMS to develop normative data for lean and obese subjects. In this specific aim we will define the components of NEAT in the lean and obese. This will not only be essential information for users of PAMS but will enhance our understanding of how physical activity and its components impacts obesity. PAMS has the potential to empower researchers from a variety of disciplines to measure physical activity precisely and cost-effectively. This should allow us to better define the role of physical activity in health and disease.